Tensor: implement abelian symmetries

[jcmgray]

Implementing symmetries allows the targeting of sub-spaces in algorithms such as DMRG, as well as affording memory and computation cost savings.

A useful reference implementation might be: https://github.com/mhauru/tensors

At the atomistic level, a AbelianTensor class would need to implement at least: transposing, reshaping, tensor-dotting, and splitting.